The present invention relates to a process for preparing novel monodisperse anion exchangers whose strongly basic functional groups are either in the center of the resin particle or in the shell of the resin particle, as well as to their use.
U.S. Pat. No. 4,444,961 discloses, inter alia, a process for preparing monodisperse anion exchangers. Here, haloalkylated polymers are reacted with an alkylamine.
EP-A 46,535 describes, with reference to U.S. Pat. No. 3,989,650, the preparation of monodisperse, macroporous strongly basic anion exchangers of uniform particle size by a direct spraying and micro-encapsulation process.
EP-A 351,621 discloses the preparation of bifunctional hetero-disperse anion exchangers whose strongly basic functional groups are distributed in the center of the resin particle and in the shell of the resin particle.
Starting from this prior art, the object of the present invention was to provide a method for preparing monodisperse anion exchangers, preferably monodisperse macroporous anion exchangers with a high mechanical and osmotic stability of the beads, with an improved absorption capacity for anions, and at the same time with the strongly basic functional groups distributed either in the center of the resin particle or in the shell of the resin particle.
The present invention provides a process for preparing monodisperse anion exchangers having strongly basic functional groups distributed either in the center of the resin particle or in the shell of the resin particle comprising
(a) reacting monomer droplets made from at least one monovinylaromatic compound and at least one polyvinylaromatic compound, and, if desired, a porogen and/or, if desired, an initiator or an initiator combination to give a monodisperse, crosslinked bead polymer,
(b) amidomethylating the monodisperse, crosslinked bead polymer from step (a) with phthalimide derivatives,
(c) converting the amidomethylated bead polymer from step (b) to an aminomethylated bead polymer,
(d) Leuckart-Wallach-alkylating the aminomethylated bead polymer from step (c) to give a weakly basic anion exchanger having tertiary amino groups,
(e) partially loading the weakly basic anion exchanger from step (d) using a strong acid, and
(f) quaternizing the partially loaded weakly basic anion exchanger from step (e).
Surprisingly, the monodisperse anion exchangers prepared according to the present invention have a higher utilizable capacity when in use, lower pressure loss, and higher osmotic and mechanical stability than the resins known from the above-mentioned prior art, in particular from EP-A 351,621.
The monodisperse, crosslinked vinylaromatic base polymer according to process step (a) may be prepared by the processes known from the literature. Processes of this type are described, for example, in U.S. Pat. No. 4,444,961, EP-A 46,535, U.S. Pat. No. 4,419,245, or WO 93/12167, the contents of which are incorporated into the present application by way of reference in relation to process step (a).
In process step (a), at least one monovinylaromatic compound and at least one polyvinylaromatic compound are used. However, it is also possible to use mixtures of two or more monovinylaromatic compounds and mixtures of two or more polyvinylaromatic compounds.
Preferred monovinylaromatic compounds for the purposes of the present invention in process step (a) are monoethylenically unsaturated compounds, such as styrene, vinyltoluene, ethylstyrene, xcex1-methylstyrene, chlorostyrene, chloromethylstyrene, alkyl acrylates, and alkyl methacrylates. Particular preference is given to the use of styrene or mixtures of styrene with the above-mentioned monomers.
Preferred polyvinylaromatic compounds for the purposes of the present invention for process step (a) are multifunctional ethylenically unsaturated compounds, such as divinylbenzene, divinyltoluene, trivinylbenzene, divinylnaphthalene, trivinylnaphthalene, 1,7-octadiene, 1,5-hexadiene, ethylene glycol dimethacrylate, trimethylolpropane trimethacrylate, or allyl methacrylate.
The amounts used of the polyvinylaromatic compounds are generally from 1 to 20% by weight (preferably from 2 to 12% by weight, particularly preferably from 4 to 10% by weight), based on the monomer or its mixture with other monomers. The nature of the polyvinylaromatic compounds (crosslinking agents) is selected with the subsequent use of the spherical polymer in mind. In many cases divinylbenzene is suitable. For most uses, commercial qualities of divinylbenzene are sufficient, and comprise ethylvinylbenzene besides the divinylbenzene isomers.
In one preferred embodiment of the present invention, micro-encapsulated monomer droplets are used in process step (a).
Possible materials for the microencapsulation of the monomer droplets are those known for use as complex coacervates, in particular polyesters, natural or synthetic polyamides, polyurethanes, and polyureas.
An example of a particularly suitable natural polyamide is gelatin, which is used in particular as coacervate and complex coacervate. For the purposes of the present invention, gelatin-containing complex coacervates are primarily combinations of gelatin with synthetic polyelectrolytes. Suitable synthetic polyelectrolytes are copolymers incorporating units of, for example, maleic acid, acrylic acid, methacrylic acid, acrylamide, or methacrylamide. Particular preference is given to the use of acrylic acid and acrylamide. Gelatin-containing capsules may be hardened using conventional hardeners, such as formaldehyde or glutaric dialdehyde. The encapsulation of monomer droplets with gelatin, with gelatin-containing coacervates and with gelatin-containing complex coacervates is described in detail in EP-A 46,535. The methods for encapsulation using synthetic polymers are known. An example of a highly suitable process is interfacial condensation, in which a reactive component dissolved in the monomer droplet (for example, an isocyanate or an acid chloride) is reacted with a second reactive component dissolved in the aqueous phase (for example, an amine).
The monomer droplets, which may be microencapsulated if desired, may, if desired, contain an initiator or mixtures of initiators to initiate the polymerization. Examples of initiators suitable for the novel process are peroxy compounds, such as dibenzoyl peroxide, dilauroyl peroxide, bis(p-chlorobenzoyl)peroxide, dicyclohexyl peroxydicarbonate, tert-butyl peroctoate, tert-butyl peroxy-2-ethylhexanoate, 2,5-bis(2-ethylhexanoyl-peroxy)-2,5-dimethylhexane, and tert-amylperoxy-2-ethylhexane, and azo compounds, such as 2,2xe2x80x2-azobis(isobutyronitrile) and 2,2xe2x80x2-azobis(2-methyl-isobutyronitrile).
The amounts used of the initiators are generally from 0.05 to 2.5% by weight (preferably from 0.1 to 1.5% by weight), based on the mixture of monomers.
To create a macroporous structure in the spherical polymer it is possible, if desired, to use porogens as other additives in the optionally microencapsulated monomer droplets. Suitable compounds for this purpose are organic solvents which are poor solvents and, respectively, swelling agents with respect to the polymer produced. Examples that may be mentioned are hexane, octane, isooctane, isododecane, methyl ethyl ketone, butanol, and octanol and isomers thereof.
The concepts xe2x80x9cmicroporousxe2x80x9d or xe2x80x9cgelxe2x80x9d and xe2x80x9cmacroporousxe2x80x9d have been described in detail in the technical literature.
Substances that are monodisperse for the purposes of the present application are those for which the diameter of at least 90% by volume or by weight of the particles varies from the most frequent diameter by not more than 10% of the most frequent diameter.
For example, in the case of a substance with a most frequent diameter of 0.5 mm, at least 90% by volume or by weight have a size range from 0.45 to 0.55 mm, and in the case of a substance with a most frequent diameter of 0.7 mm, at least 90% by weight or by volume have a size range from 0.77 mm to 0.63 mm.
Bead polymers preferred for the purposes of the present invention and prepared in process step (a) have a macroporous structure.
Monodisperse macroporous bead polymers may be produced, for example, by adding inert materials (porogens) to the monomer mixture during the polymerization. Suitable substances of this type are primarily organic substances that dissolve in the monomer but are poor solvents and, respectively, swelling agents for the polymer (precipitants for polymers), for example, aliphatic hydrocarbons (Farbenfabriken Bayer DBP 1045102, 1957; DBP 1113570, 1957).
U.S. Pat. No. 4,382,124, for example, uses alcohols having from 4 to 10 carbon atoms as porogen for preparing monodisperse, macroporous bead polymers based on styrene/divinylbenzene. An overview of preparation methods for macroporous bead polymers is also given.
The monomer droplets, which may be microencapsulated if desired, may also, if desired, comprise up to 30% by weight (based on the monomer) of crosslinked or non-crosslinked polymer. Preferred polymers derive from the above-mentioned monomers, particularly preferably from styrene.
The average particle size of the monomer droplets, which may be encapsulated if desired, is from 10 to 1000 xcexcm, preferably from 100 to 1000 xcexcm. The novel process is also very suitable for preparing monodisperse spherical polymers.
When monodisperse bead polymers are prepared according to process step (a) the aqueous phase may, if desired, comprise a dissolved polymerization inhibitor. Both inorganic and organic substances are possible inhibitors for the purposes of the present invention. Examples of inorganic inhibitors are nitrogen compounds, such as hydroxylamine, hydrazine, sodium nitrite, and potassium nitrite, salts of phosphorous acid, such as sodium hydrogen phosphite, and sulfur-containing compounds, such as sodium dithionite, sodium thiosulfate, sodium sulfite, sodium bisulfite, sodium thiocyanate, and ammonium thiocyanate. Examples of organic inhibitors are phenolic compounds, such as hydroquinone, hydroquinone monomethyl ether, resorcinol, pyrocatechol, tert-butyl-pyrocatechol, pyrogallol, and condensation products made from phenols with aldehydes. Other suitable organic inhibitors are nitrogen-containing compounds, including hydroxylamine derivatives, such as N,N-diethyl-hydroxylamine, N-isopropylhydroxylamine, and sulfonated or carboxylated derivatives of N-alkylhydroxylamine or of N,N-dialkylhydroxylamine, hydrazine derivatives, such as N,N-hydrazinodiacetic acid, nitroso compounds, such as N-nitrosophenylhydroxylamine, the ammonium salt of N-nitrosophenylhydroxylamine, or the aluminum salt of N-nitrosophenyl-hydroxylamine. The concentration of the inhibitor is from 5 to 1000 ppm (based on the aqueous phase), preferably from 10 to 500 ppm, particularly preferably from 10 to 250 ppm.
As mentioned above, the polymerization of the optionally micro-encapsulated monomer droplets to give the spherical monodisperse bead polymer may, if desired, take place in the presence of one or more protective colloids in the aqueous phase. Suitable protective colloids are natural or synthetic water-soluble polymers, such as gelatin, starch, polyvinyl alcohol, polyvinylpyrrolidone, polyacrylic acid, polymethacrylic acid, or copolymers made from (meth)acrylic acid and from (meth)-acrylates. Other very suitable materials are cellulose derivatives, particularly cellulose esters and cellulose ethers, such as carboxymethylcellulose, methylhydroxyethylcellulose, methylhydroxypropylcellulose, and hydroxyethylcellulose. Gelatin is particularly suitable. The amount used of the protective colloids is generally from 0.05 to 1% by weight (preferably from 0.05 to 0.5% by weight), based on the aqueous phase.
The polymerization to give the spherical, monodisperse bead polymer in process step (a) may, if desired, also be carried out in the presence of a buffer system. Preference is given to buffer systems that set the pH of the aqueous phase at the beginning of the polymerization to between 14 and 6 (preferably between 12 and 8). Under these conditions protective colloids having carboxylic acid groups are present to some extent or entirely in the form of salts, which has a favorable effect on the action of the protective colloids. Particularly suitable buffer systems comprise phosphate salts or borate salts. For the purposes of the present invention, the terms phosphate and borate also include the condensation products of the ortho forms of corresponding acids and salts. The concentration of the phosphate or borate in the aqueous phase is from 0.5 to 500 mmol/l, preferably from 2.5 to 100 mmol/l.
The stirring speed during the polymerization is relatively non-critical and, unlike in conventional bead polymerization, has no effect on the particle size. The stirring speeds used are low speeds which are sufficient to keep the monomer droplets in suspension and to promote dissipation of the heat of polymerization. A variety of stirrer types can be used for this task. Gate stirrers with an axial action are particularly suitable.
The ratio by volume of encapsulated monomer droplets to aqueous phase is from 1:0.75 to 1:20, preferably from 1:1 to 1:6.
The polymerization temperature depends on the decomposition temperature of the initiator used and is generally from 50 to 180xc2x0 C. (preferably from 55 to 130xc2x0 C.). The polymerization takes from 0.5 hour to a few hours. It has proven successful to use a temperature program in which the polymerization is begun at a low temperature (for example, 60xc2x0 C.) and the reaction temperature is raised as the polymerization conversion progresses. This is a very good way of fulfilling, for example, the requirement for a reaction that proceeds reliably and with a high polymerization conversion. After polymerization, the polymer is isolated using conventional methods (for example, by filtration or decanting) and washed if desired.
In process step (b) the amidomethylating reagent is first prepared. This is done, for example, by dissolving a phthalimide or a phthalimide derivative in a solvent and mixing with formalin. A bis(phthalimido)ether is then formed from this material with elimination of water. The bis(phthalimido)ether may, if desired, be reacted to give the phthalimido ester. For the purposes of the present invention, preferred phthalimide derivatives are phthalimide itself and substituted phthalimides such as methylphthalimide.
Solvents used in process step (b) are inert and suitable for swelling the polymer and are preferably chlorinated hydrocarbons, particularly preferably dichloroethane or methylene chloride.
In process step (b) the bead polymer is condensed with phthalimide derivatives. The catalyst used here comprises oleum, sulfuric acid, or sulfur trioxide.
Process step (b) is carried out at temperatures of from 20 to 120xc2x0 C., preferably from 50 to 100xc2x0 C., particularly preferably from 60 to 90xc2x0 C.
The elimination of the phthalic acid residue, and with this the release of the aminomethyl group, takes place in process step (c) via treatment of the phthalimidomethylated crosslinked bead polymer with aqueous or alcoholic solutions of an alkali metal hydroxide, such as sodium hydroxide or potassium hydroxide, at temperatures of from 100 to 250xc2x0 C. (preferably from 120 to 190xc2x0 C.). The concentration of the aqueous sodium hydroxide is from 10 to 50% by weight, preferably from 20 to 40% by weight. This process allows the preparation of crosslinked bead polymers containing aminoalkyl groups with substitution of the aromatic rings at a level greater than 1.
The resultant aminomethylated bead polymer is finally washed with deionized water until free of alkali.
In process step (d) the anion exchangers are prepared by reacting the aminomethylated monodisperse, crosslinked vinylaromatic base polymer in suspension with Leuckart-Wallach alkylating agents to give weakly basic anion exchangers having tertiary amino groups. Leuckart-Wallach reagents are described, by way of example, in Organikum [Organic Chemistry], VEB Deutscher Verlag der Wissenschaften, Berlin 1968, 8th Edition, page 479.
Water is used as suspension medium.
Process step (d) is carried out at temperatures of from 20 to 150xc2x0 C. (preferably from 40 to 110xc2x0 C.) and at pressures of from atmospheric pressure to 6 bar (preferably at from atmospheric pressure to 4 bar).
There are various ways of undertaking the loading with strong acids according to process step (e) prior to quaternization:
1. by directly partially loading the weakly basic anion exchanger with the calculated amount of acid; or
2. by undertaking the partial loading in two stages by first fully loading the weakly basic anion exchanger with an excess of acid and in a second stage partially regenerating the fully loaded weakly basic anion exchanger by treatment with a calculated amount of aqueous base (from 0.7 to 0. 15 equivalents of base per mole of amino groups in the weakly basic anion exchanger).
The quaternization of the weakly basic anion exchangers that are partially loaded according to procedure 1 gives bifunctional anion exchangers that have no isomerization action on glucose or have a significantly lower level of action than bifunctional anion exchangers having the same content of strongly basic groups prepared by known processes.
The quaternization of the weakly basic anion exchangers that are partially loaded in two stages according to procedure 2 gives bifunctional anion exchangers that, compared with bifunctional anion exchangers of the same degree of quaternization and prepared by known processes, have better decolorizing performance, are easier to regenerate, and have a still lower tendency toward contamination by organic substances.
It has been found that partial preloading of the weakly basic anion exchange resins using strong acids gives a certain arrangement of the strongly and weakly basic groups within the resin particle and that this certain arrangement of the strongly basic groups within the resin particle is the source of the new and improved properties of the bifunctional, monodisperse anion exchangers having strongly basic functional groups and obtainable according to the invention.
The quaternization of the weakly basic anion exchangers partially loaded according to procedure 1 gives bifunctional anion exchangers which contain strongly basic groups in the center of the resin particle; when the weakly basic anion exchangers partially loaded according to procedure 2 are quaternized, the strongly basic groups are in the shell of the resin particle. Quaternization of the weakly basic anion exchangers without acid pretreatment gives bifunctional anion exchangers in which the strongly basic groups have a random distribution across the entire cross section of the particle.
In order to achieve the best possible uniformity of partial loading of the resin particles, the partial loading of the weakly basic anion exchangers using acids according to procedure 1 is preferably undertaken by suspending the weakly basic anion exchanger in the calculated amount of aqueous acid and intensively stirring the suspension at temperatures of from 5 to 40xc2x0 C., preferably at room temperature, until the pH of the aqueous solution shows no further change. However, the partial loading of the weakly basic anion exchanger using acid may also be undertaken by suspending the exchanger in deionized water and mixing the suspension with the calculated amount of acid, with intensive stirring at temperatures of from 5 to 40xc2x0 C., and then again stirring to constant pH. The partial quaternization to give the bifunctional anion exchanger may be undertaken directly by adding the alkylating agent to the suspension.
In the partial loading of the weakly basic anion exchangers according to procedure 2 the first substage (i.e., the full loading of the weakly basic anion exchanger using acids) may take place either by suspending the weakly basic anion exchanger in the acid and stirring the suspension to constant pH of the aqueous solution (batch process) or by passing the acid over the anion exchanger in a filter column (column process). After the full loading the excess of acid is removed by washing with deionized water. The second substage (i.e., the partial regeneration) is preferably undertaken by a batch process in order to achieve the greatest possible uniformity of partial regeneration of all of the resin particles. That is, the weakly basic anion exchanger in salt form is suspended in deionized water and the suspension mixed with the calculated amount of base, with intensive stirring at temperatures of from 5 to 40xc2x0 C., and then stirred to constant pH of the aqueous solution.
Acids suitable for use for the loading of the weakly basic anion exchangers are strong inorganic acids, such as hydrochloric acid, nitric acid, sulfuric acid, or phosphoric acid, and strong organic acids, such as formic acid or p-toluenesulfonic acid. The inorganic acids are preferred for cost reasons.
The concentration of the acids in the aqueous solutions used for the loading is preferably from 0.1 to 20% by weight, in particular from 5 to 10% by weight.
The bases used as aqueous solutions for the partial regeneration of the fully loaded weakly basic anion exchangers may be either inorganic or organic. Preference is given to the use of aqueous solutions of sodium hydroxide, potassium hydroxide, sodium carbonate, or ammonia. The concentration of the bases in the aqueous solutions is preferably from 5 to 10% by weight.
The weakly basic anion-exchange resins to be used in the novel process for preparing the bifunctional anion exchangers are known, as is their preparation. See, for example, Ullmanns Enzyklopxc3xa4die der technischen Chemie [Ullmann""s Encyclopaedia of Industrial Chemistry], 4th Edition, Vol. 13, pages 301 to 303. It is possible to use weakly basic anion-exchange resins based on crosslinked polyacrylates or on crosslinked polystyrenes. The anion exchangers may be gels or macroporous. Bifunctional anion exchangers with particularly advantageous properties are obtained starting from resins made from crosslinked polystyrene.
Following the partial loading according to the invention of the weakly basic anion exchangers using strong acids, the anion exchangers, partially in salt form, are quaternized in a conventional manner. See Ullmanns Enzyklopxc3xa4die [Ullmann""s Encyclopaedia], cited above. The degree of quaternization depends on the application for which the particular anion exchanger is intended. For deionizing sugar solutions, it is preferable to use bifunctional anion exchangers based on crosslinked polystyrene and having a content of from 5 to 25% of strongly basic groups, based on all of the basic groups present in the anion exchanger. For decolorizing sugar solutions it is preferable to use bifunctional anion exchangers quaternized using hydrophobic radicals (e.g., benzyl radicals) and based on crosslinked polyacrylate, and having a content of strongly basic groups of from 20 to 75% (preferably from 40 to 80%), based on all of the basic groups present in the anion exchanger.
The present invention also provides the monodisperse anion exchangers prepared according to the novel process and having strongly basic functional groups distributed either in the center of the resin particle or in the shell of the resin particle.
The novel process preferably gives monodisperse anion exchangers having the functional groups 
wherein
R1 is an alkyl group, a hydroxyalkyl group, or an alkoxyalkyl group,
R2 is an alkyl group, an alkoxyalkyl group, or a hydroxyalkyl group,
R3 is an alkyl group, an alkoxyalkyl group, or a hydroxyalkyl group,
n is an integer from 1 to 5 (particularly preferably 1), and
X is an anionic counterion (preferably Brxe2x88x92, Clxe2x88x92, SO42xe2x88x92, NO3xe2x88x92 or OHxe2x88x92), where group (2) is either very particularly preferably in the center of the resin particle or very particularly preferably in the shell of the resin particle.
In the radicals R1, R2, and R3 it is preferable for each alkoxy and alkyl to contain from 1 to 6 carbon atoms.
Each aromatic ring in the novel monodisperse anion exchangers preferably has from 0.1 to 2 of the above-mentioned functional groups (1) or (2).
The anion exchangers prepared according to the invention are used
to remove anions from aqueous or organic solutions or their vapors,
to remove anions from condensates
to remove color particles from aqueous or organic solutions or their vapors,
to decolorize and deionize wheys, aqueous gelatin solutions, fruit-juices, fruit-musts, and aqueous solutions of sugars (preferably mono- and disaccharides, especially preferably aqueous solutions of glucose, fructose, aqueous solutions of sugar from sugar canes or sugar beets) in the sugar industry, starch industry, pharmaceutical industry, or dairy farms, and
to remove organic components from aqueous solutions, for example, humic acids from surface water.
The novel anion exchangers may also be used for purifying and treating water in the chemical industry or electronics industry, in particular for preparing ultra-high-purity water.
The novel anion exchangers may also be used in combination with gel-type and/or macroporous cation exchangers for deionizing aqueous solutions and/or condensates, in particular in the sugar industry.